$ E = \left[\begin{array}{rrr}1 & 1 & 0 \\ 1 & 2 & 0 \\ 0 & 0 & 1\end{array}\right]$ What is $ E^{-1}$ ?
$ = \left[\begin{array}{rrr}2 & -1 & 0 \\ -1 & 1 & 0 \\ 0 & 0 & 1\end{array}\right]$